This type of stratified scavenging two-cycle engine conventionally has a scavenging flow passage for connecting a cylinder chamber to a crank chamber; with a mixture flow passage, for supplying a fuel mixture, being connected to the crank chamber; and with an air flow passage, for supplying air, being connected to the scavenging flow passage. A scavenging port of the scavenging flow passage, and an exhaust port of an exhaust pipe are opened to the cylinder chamber. The aforesaid air flow passage is provided with a lead valve (a check valve) 80, shown in FIG. 12, for only allowing the air to flow toward the scavenging flow passage.
In the stratified scavenging two-cycle engine configured as above, a piston 3 ascends, thereby starting to reduce the pressure inside a crank chamber 20 and to increase the pressure inside the cylinder chamber 10; and as the piston 3 ascends, a scavenging port 81 and an exhaust port are sequentially closed. In this situation, a mixture flows into the crank chamber 20 with the pressure therein being reduced, and air from an air flow passage 83 pushes the lead valve 80 open to flow therein through a scavenging flow passage 85.
When the piston 3 reaches the vicinity of the top dead center, the mixture in the cylinder chamber 10 is ignited, and thereafter the piston 3 descends. The piston 3 descends, thereby starting to increase the pressure inside the crank chamber 20; and while the piston 3 is descending, the exhaust port and the scavenging port 81 are sequentially opened, and combustion gas is exhausted via the exhaust port. Subsequently, when the scavenging port 81 is opened, the air remaining in the scavenging flow passage 85 bursts out into the cylinder chamber 10 due to the pressure inside the crank chamber 20. As a result, the combustion gas remaining in the cylinder chamber 10 is expelled. Subsequently, the mixture in the crank chamber 20 is charged into the cylinder chamber 10 through the scavenging flow passage 85. Again, when the piston 3 starts to ascend from the bottom dead center, the pressure inside the crank chamber 20 starts to reduce, and the cycle as described above is repeated once again.
According to the stratified scavenging two-cycle engine configured as above, the inside of the cylinder chamber 10 can be initially scavenged by air, thereby making it possible to prevent the combustible gas from being discharged by the blow-by of the mixture, which provides the advantage that the exhaust gas becomes clean.
However, in the aforesaid stratified scavenging two-cycle engine, as shown in FIG. 12, the air flowing into the scavenging flow passage 85 from the lead valve 80 does not flow into a space 81A in the vicinity of the scavenging port 81, and therefore mixture remains in this space. There exists a disadvantage in that the mixture, together with the air remaining in the scavenging flow passage 85, is discharged from the exhaust port into the atmosphere with the combustion gas via the cylinder chamber 10 when the scavenging port 81 opens in the exhaust stroke in which the piston 3 descends. In addition, the lead valve 80 is provided in the air flow passage 83, thereby causing a disadvantage in that the lead valve 80 becomes intake resistance when air is taken into the scavenging flow passage 85. Further, the number of components is increased due to the lead valve 80, and the structure is complicated, thus causing the disadvantage of increased costs.